The present invention relates to a vehicle-mounted charging apparatus for portable telephone sets used as a public communication device.
FIG. 2 shows the structure of a conventional vehicle-mounted charging apparatus for a portable telephone set. In FIG. 2, reference numeral 21 denotes a portable telephone set to which a battery pack 22 containing a battery 22a is made connectable. Reference numeral 23 denotes a power source switch which is switched on/off by an internal circuit 24. Reference numeral 25 denotes a temperature sensor such as a thermistor which transmits information on sensed temperatures in the battery pack 22 through an external connector 26 and a connection cable 27, which has connectors 27a and 27b attached to its respective ends, to a vehicle-mounted charger 28 through an external connector 211 thereof.
Reference numeral 29 denotes a constant current drive transistor provided in the charger 28 and having an emitter impressed with a voltage Vcc from a vehicle-mounted battery (not shown) and a collector from which a charging current is supplied through a reverse current preventing diode 30 to the battery 22a in the battery pack 22. Reference numeral 210 denotes a constant current control microcomputer which supplies a signal to the base of the transistor 29 to control the operation of the transistor 29. Reference numeral 211 denotes an external connector provided in the charger 28 for connection to the connector 27b of the connection cable 27. Thus, the charging apparatus 28 is composed of the transistor 29, diode 30, microcomputer 210 and external connector 211.
The operation of the conventional apparatus will next be described. In FIG. 2, the battery pack 22 is connected to the portable telephone set 21. The connectors 27a and 27b are connected to the external connectors 26 and 211, respectively, to connect the battery pack 22 and the charger 28 with each other. Then, the voltage Vcc is applied from the vehicle-mounted battery to the emitter of transistor 29 to thereby start charging of the battery 22a in the battery pack 22 through the collector of the transistor 29, diode 30, connectors 211 and 27b, connection cable 27, and connectors 27a and 26. In this case, since floating charging is performed, charging of the battery 22a continues regardless of whether the power source switch 23 of the portable telephone set 21 is switched on or off. Then, the microcomputer 210 monitors the state of the charging output terminal of the transistor 29, detects an error voltage -.DELTA.V with respect to the rated constant voltage appearing close to the time of completion of charging of a Ni--Cd or nickel--hydrogen battery to thereby terminate the floating charging and shifts to trickle charging.
In order to protect the battery, the temperature sensor 25 is provided to monitor the ambient temperature and the heat generated by the battery. At a temperature outside an appropriate charging temperature range, the microcomputer 210 is set to control the charging of the battery to be shifted to trickle charging.
In this way, in the conventional charging apparatus, floating charging in the portable telephone set is possible so far as it is performed within an appropriate charging temperature range.
However, in the conventional charging apparatus, a charging temperature range, within which rapid charging is possible, is fixed. Therefore, even when a charging current equivalent to a rapid charging current is made to flow outside the fixed charging temperature range at the time of communication of the portable telephone set, the charging apparatus operates to supply only a trickle current. As a result, the remaining current is discharged from the battery pack 22. Thus, when the battery pack 22 has become substantially empty, the portable telephone set 21 cannot be put to use.